An Introduction to Organic Reactions and Their Mechanism

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Chapter 3

• An Introduction to Organic Reactions and Their
  Mechanism
• Acids and Bases

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3.4 Acid Strength

• Strong acids completely ionized or completely
  dissociated
• Forward reaction is predominated
• Most of HA is dissociated
• Conjugated base is weak and has low attraction
  for proton
• HA(aq) H2O(l) ? H3O(aq) A-(aq)

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Acids Strength

• Weak acids partially ionized or dissolved
• Reverse reactions is predominated
• Most of HA is undissociate
• Conjugated base is strong and has high attraction
  for proton
• HC2H3O4(aq) H2O(l) H3O(aq)
  C2H3O4-(aq)

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Acid Strength
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3.5 The Strength of Acids and Bases Ka and pKa

• The acidity constant, Ka
• a quantitative measure of the strength of an acid
  in solution.
• It is known as the equilibrium constant for a
  chemical reaction known as dissociation in the
  context of acid-base reactions
• Larger Ka, stronger the acid

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Acidity and pKa

• Logarithm scale of Ka with the base of 10
• pKa - log Ka
• The larger pKa, the smaller extent of
  dissociation
• Weaker acids

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Relative Strength of selected acids and their
conjugated bases
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Examples

• An acid (HA) has Ka 10-7, what is its pKa?
• Another acid (HB) has Ka 5, what is its pKa?
• Which is the stronger acid?

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Predicting the Strength of Bases

• The strength of acids will decide the how weak or
  strong its conjugated bases
• Stronger the acids, the weaker will be its
  conjugated base
• After losing a proton, acid will become a
  conjugated base
• When a base accepts a proton, the resulting
  chemical is called the conjugate acid of that
  original base

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Acid and its conjugated base
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Examples

• The pKa of anilinium ion (C6H5NH3) is equal to
  4.6. On the basis of this fact, decide whether
  aniline (C6H5NH2) is a stronger or weak base than
  methylamine

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3.6 Predicting the Outcome of Acid-Base Reactions

• Acid-base reactions always favor the formation of
  the weaker acid and weaker base
• Equilibrium favor the formation of the most
  stable (lowest potential energy) species
• Stronger acid stronger base ? weaker base
  weaker acid

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Example

• Consider the mixing of an aqueous solution of
  acetic acid, CH3CO2H (see Table 3.1), and NaOH.
  What acid base reaction, if any would take place
• H2SO4(aq) NH3(aq) ? NH4(aq) HSO4-(aq)

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3.6 Water Solubility as the Result of Salt
Formation

• Recall Longer chain of carbon (4 or more) will
  be less soluble
• Higher molecular weight of carboxylic compounds
  do not dissolve in water
• Instead, they dissolved in a strong base to form
  a water-soluble salt

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Water Solubility

• We can predict that an amine will react with
  aqueous hydrochloric acid

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Water Solubility

• Methylamine and most amines of low molecular
  weight are very soluble in water
• Formed a water-soluble salt from hydrochloric acid

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3.7 Relationships between structure and Acidity

• The strength of a Bronsted-Lowry acid depends on
  the extend to which a proton can be separated
  from it and transferred to a base.
• Removing H ? breaking a bond
• Making the conjugated base more electronegative
• Accepting H ? forming a bond
• Making the conjugated acid less electronegative

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3.7 Relationships between structure and Acidity

• Bond strength to proton decreases as we move down
  to column
• Decreasing effectiveness of orbital overlap
  between the hydrogen 1s orbital and the orbitals
  of successively larger element in the column
• Less effective the orbital overlap, the weaker
  bond, stronger acid
• Acidity increases from left to right

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3.7 Relationships between structure and Acidity
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3.7 Relationships between structure and Acidity
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3.7 The Effect of Hybridization
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3.7 The Effect of Hybridization

• Electrons of 2s orbtials have lower energy than
  those of 2p orbitals because these electrons are
  much closer to the nucleus
• Having more s orbitals means that electrons of
  the anion will be lower in energy and more stable

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3.7 The Effect of Hybridization

• sp C sH hybridization
• Contain 50 s character
• 1s 1p
• More electronegative spC

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3.7 The Effect of Hybridization

• sp2C sH hybridization
• Contains 33.3 s character
• 1s 2p
• Less electronegative than spC

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3.7 The Effect of Hybridization

• sp3C-sH hybridization
• Contains 25 s character
• 1s 3p
• Least electronegative C

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3.7 The Effect of Hybridization
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3.7 The Effect of Hybridization

• Relative basicity of the carbanions
• Ethynide ion is the weakest base
• The more electronegative carbon, the more stable
  the anion